Prevent random behavior in case of file corruption (fNbytesFree)
Prevent random behavior in case of file corruption (fNbytesFree)

This fixes ROOT-6240

kirbyherm/root-r-tools,tc3t/qoot,kirbyherm/root-r-tools,kirbyherm/root-r-tools,tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,kirbyherm/root-r-tools,kirbyherm/root-r-tools,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,tc3t/qoot,tc3t/qoot